Leptospira interrogans (serovars Pomona, Icterohaemorrhagiae, Canicola, Hardjo, and Bratislava), L borgpetersenii (serovars Sejroe and Tarassovi), and Leptospira kirschneri (serovar Grippotyphosa) are all reported to infect pigs. Serovars Pomona and Bratislava are uniquely adapted to swine; others are maintained in other species but sometimes infect swine. Swine are maintenance hosts for serovar Bratislava, and infected pigs rarely develop signs typical of acute leptospirosis—rather, reproductive failure as evidenced by infertility and sporadic abortion is the most common clinical sign, and venereal transmission may occur. Serovar Pomona, in contrast, is of intermediate pathogenicity for swine, with acute clinical signs seen in young pigs and abortions (often in groups) occurring in pregnant swine. Although Pomona infections are associated with acute, sometimes severe clinical signs suggestive of an incidental host infection, pigs often remain infected and shed serovar Pomona for weeks to a few months after infection. This feature of Pomona infection can be associated with high rates of pig-to-pig transmission among swine reared in confinement. Incidental infections may occur from strains belonging to the Grippotyphosa, Icterohaemorrhagiae, and Canicola serogroups.
Abortions occurring 2–4 wk before term are the most common manifestation of leptospirosis in pigs. Piglets produced at term may be dead or weak and may die soon after birth. The principal differential diagnosis is porcine reproductive and respiratory syndrome (see Porcine Reproductive and Respiratory Syndrome), although brucellosis, parvovirus, and SMEDI (stillbirth, mummification, embryonic death, and infertility) share some features with leptospirosis. Acute leptospirosis, as described in calves, has been described in piglets but is rare. Treatment and control are similar to those described for cattle, using a combination of medication either to prevent infection or to decrease shedding, vaccination, rodent and small mammal control, and feed and water free of Leptospira organisms. Immunization through use of bacterins is widely practiced in breeding herds and will reduce the prevalence of infection and abortions. The bacterin must be serovar specific for protection. Bacterins should not be expected to eliminate infection in carriers.